CHLOR-ALKALI MEMO
Overview
1 Complete / choose from the options to summarise the purpose and process of the
chlor-alkali industry.
Input: Sodium chloride solution (brine)
+
-
attracted
ANODE:
[oxidation/reduction]
[positive / negative] ions
CATHODE:
[oxidation/reduction]
Power Source
[Electrolytic / voltaic] cell
Converts electrical energy chemical energy
as a potential difference, placed across two electrodes, forces a
[spontaneous/non-spontaneous] chemical reaction to occur.
Outputs:
Chlorine : Cl2 (g)
Sodium hydroxide : NaOH (aq)
Uses:
Bleach
Soap-making
Disinfectant
Water purification
Used to make PVC Drain cleaning
Hydrogen: H2 (g)
Rocket fuel
Possible alternative car fuel
Needed for making plastics
2 Complete / choose from the options to summarise the types of chlor-alkali cells.
Cathode material
Membrane
Diaphragm
Mercury
carbon
carbon
mercury
Input
brine (NaCl(aq)) and an electric potential difference (PD)
Oxidation  reaction at anode
2Cl - (aq)
Cl2 + 2e -
Reduction  reac- H O( l) reduced to H (g)+OH - (aq)
2
2
tion at cathode
Since there are Na+ ions present too, NaOH can be
produced.
Na+ reduced to Na, which then
reacts with H2O to form NaOH
+ H2
What divides the
electrolyte into
two half cells?
a semi-permeable diaphragm
reduction  reaction occurs
inside the liquid mercury
cathode
diaphragm allows Cl - ions into
the reduction  cell,
contaminating the product with
significant amounts of NaCl
mercury is poisonous
a semi-permeable
membrane
Disadvantages
3 Choose from the options for an electrolytic cell.
ELECTROLYTIC CELL
Anode
Cathode
[oxidation / reduction]
[oxidation / reduction]
[positive / negative]
[positive / negative]
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4 Circle the correct option (True / False) for each of the following referring to an electrolytic cell.
a A battery makes one electrode positive and the other negative, and this causes a
chemical reaction to occur. [True / False]
b A chemical reaction occurs, and this causes one electrode to be made positive and the
other negative. [True / False]
c As oxidation happens, chemicals lose electrons, which then go onto the anode,
making it positive. [True / False]
d The battery makes the anode positive, and this makes chemicals lose electrons there as
the battery sucks electrons away from them, causing oxidation to happen. [True / False]
e As reduction happens, chemicals accept electrons, taking them from the cathode,
causing it to become negative.
[True / False]
f The battery makes the cathode negative, and this makes chemicals gain electrons
there, causing reduction to happen. [True / False]
5 Link each element from Column A with its corresponding element in Column B. Write the letter from A next to each item in B in the last column.
Column A
a anode
b cathode
c electrode
d electrolyte
e oxidation
f reduction
g
h
i
j
k
l
m
n
redox
soap
hydrophilic
hydrophobic
bleach
chlorine (Cl2)
sodium hydroxide
disinfectant
Column B
colour-remover
a site at which the redox reactions of an electrochemical cell
occur; often a rod
a half reaction in which one chemical loses electrons, donating
them to another chemical
a half reaction in which one chemical gains electrons from
another chemical
an electrode at which oxidation occurs
a kind of reaction in which electrons are transferred from a
reducing agent to an oxidising agent
attracted to water
an ionic solution, therefore able to conduct electricity
repelled by water
an electrode at which reduction occurs
an organic chemical which is able to make fat dissolve
a strong alkali; can be used to make soap; caustic soda
a substance which kills germs
a dense green gas useful as bleach and disinfectant
A
k
c
e
f
a
g
i
d
j
b
h
m
n
l
Membrane Cell
6 Complete the explanation by filling the gaps
or choosing from the options.
Do this before, or after, but not
during, watching the animations.
Mark during re-watching.
Cell composition
The membrane cell is a(n) [electrolytic/voltaic]
cell. It consists of an external power supply (a
battery), two electrodes, and electrolytes. The
cell is divided into two halves. The two halves are
separated by a semi-permeable membrane. Semipermeable means it only allows certain ions
through.
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Electrolyte
Brine serves as the electrolyte for the anode's half reaction. An electrolyte is a solution which conducts
electricity. Brine consists of sodium chloride dissolved in water. When sodium chloride dissolves in
water, its positively charged [sodium/chloride] ions are separated from its negatively charged [sodium/
chloride] ions, and each ion is surrounded by water.
Anode
The battery creates a potential difference across the electrodes. The positive terminal of the battery
[pulls electrons away from/pushes electrons into] the electrode connected to it, charging the
electrode [positively/negatively]. This attracts the [positively/negatively/neutrally] charged [sodium/
chloride] ions in the brine. When these reach the electrode, the battery [pulls an electron away
from/pushes electrons into] each of them. This converts them from [positively/negatively/neutrally]
charged chloride ions into [positive/negative/neutral] chlorine atoms. These are unstable, so they bond
covalently with one another in pairs, forming diatomic molecules of Cl2. This is what we often mean
when we refer to chlorine. Chlorine is a dense green gas which bubbles away from this electrode
and is collected. Chlorine was formed as chloride ions were stripped of electrons, so we call this half
reaction [oxidation/reduction], which is the [gain/loss] of electrons. Each [chlorine atom/chloride ion]
loses one electron to change it into a [chlorine atom/chloride ion]. The electrode at which oxidation
occurs is called the [cathode/anode]. In an electrolytic cell, such as this, the battery charges the anode
[positively/negatively].
Cathode
The semi-permeable membrane allows [positively/negatively] charged sodium [ions/atoms] through
into the other electrolytic half-cell. The electrode in this half-cell is connected to the negative terminal
of the battery. The battery [pulls electrons away from/pushes electrons into] it, charging the electrode
[positively/negatively/neutrally]. This causes water molecules to react at this electrode. The water
molecules [lose/accept] the electrons. We say the water is [oxidised/reduced]. These electrons cause
water to change into hydrogen gas (H2) and [positively/negatively/neutrally] charged hydroxyl ions
(OH-). These hydroxyl ions are dissolved in water, as are the sodium ions which moved through the
membrane. Sodium ions and hydroxyl ions are therefore present in the solution. We call this a sodium
hydroxide solution. The electrode at which reduction occurs is called the [cathode/anode]. In an
electrolytic cell, such as this, the battery charges the cathode [positively/negatively].
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